1. Field of the Invention
This invention relates to a direct fuel injection system for an engine, and more particularly to an improved direct fuel injection system that protects the fuel injectors from heat.
2. Description of Related Art
In all fields of engine design, there is an increasing emphasis on obtaining more effective emission control, better fuel economy and, at the same time, continued high or higher power output. This trend has resulted in the substitution of fuel injection systems for carburetors as the engine charge former.
Fuel injection systems typically inject fuel into the air intake manifold. However, in order to obtain still better engine performance, direct injection systems are being considered. Direct fuel injection systems inject fuel directly into the combustion chamber. These systems potentially have significant advantages over typical fuel injection systems such as improved emission control.
In a direct injection system, a fuel injector is typically positioned in a cavity that is defined by a cylinder head. The nozzle of the fuel injector is exposed to the combustion chamber through an opening extending from the cavity so that the fuel may be injected directly into the combustion chamber. Usually, the inner diameter of the opening is smaller than the inner diameter of the cavity so that a step is formed between the opening and the cavity. The body of the fuel injector is seated at the step. A sealing member such as, for example, a disc spring is typically interposed between the injector body and the step so as to seal the combustion chamber.
Because the injector nozzle is exposed to the combustion chamber in which the injected fuel bums at an extremely high temperature, the fuel injector is exposed to considerable heat. Also, the sealing member may be insufficient to prevent the combustion flame from reaching the injector body. Additionally, the cylinder head itself is typically very hot and therefore heat conducts to the fuel injector from the cylinder head through the sealing member or directly to the fuel injector where the fuel injector contacts the cylinder head. The fuel injector is partially cooled by the injected fuel especially before the combustion stroke; however, this cooling effect is typically insufficient.
Accordingly, the fuel injector is exposed to a significant amount of heat that is conducted to the nozzle. This can causes significant problems. For example, a certain amount of liquid fuel that contains heavy oil components typically exists on the injector nozzle immediately after injection. If the heat in the injector nozzle exceeds the distillation temperature of the liquid fuel (for example, 90% of gasoline components evaporate at around 150.degree.), the heavy oil components tend to deposit on the injector nozzle. Excessive deposits of the heavy oil components on the nozzle can cause deformations of the injector shape. Deformation of the injector shape can decrease or cause fluctuations in the amount of fuel injected into the engine. Accordingly, the fuel/air ratio may deviate from optimum conditions for the control of emissions. This phenomenon is particularly a problem for marine engines, which are typically two stroke engines, because combustion occurs at every stroke of the piston. Furthermore, marine engines are typically used at high loads and high engine speeds.